The present invention concerns an improved tool for interconnecting sheet webs by welding, the tool being of the kind comprising two opposite jaws on which are supported means for clamping and cutting the sheet webs and welding them together.
Several kinds of tools are already known for welding together a sheet web enclosing one or several articles. Usually, one jaw of the tool is stationary while the opposite jaw is formed by two collet jaws and by welding heads positioned internally thereof, with some cutting means, e.g. a knife, positioned therebetween.
In accordance with a common, previously known method the welding heads are heated by means of heat wires positioned on the side of the welding heads facing the opposite jaw. To prevent the sheet web parts from adhering or sticking to the welding heads during the welding moment proper the welding wires are covered by a cloth, preferably made from Teflon plastics.
However, a tool constructed in this manner suffers from several disadvantages. The heat wires rapidly wear out and thus must be replaced frequently. This is true also of the Telfon cloth covering the wires. The Teflon plastics, suitable per se for the purpose in view in this connection, still has the disadvantage of being a poor heat conductor. Conssequeantly, the cloth must be thin in order not or prevent heat transfer to the sheet webs to too large an extent and as a result also the cloth wears out rapidly. To eliminate too frequent shut-downs, means have been provided to advance the Teflon cloth successively as it is worn out past the heat wires. Naturally, this arrangement means that the tool becomes more expensive.
Also other disadvantages exist. Although heat wires are used instead of homogeneously heated welding heads it is difficult to insulate the cutting means, for instance a knife, positioned between the welding heads and prevent it from being heated. If the knife becomes too warm it will, however, stick to the sheet web portions during the severing moment, resulting in poor cutting efficiency. In addition, it is not possible to obtain an indication of the temperature of the heat wires directly at the place of welding, which arises problems of regulating the temperature to achieve the best possible welding result.